The conventional lock for a video tape cassette has a complicated constitution as shown in FIG. 5. A base plate 4 is provided with a lock-releasing button 1 and a locking protrusion 1', which are inserted into a hole 2' formed in the side wall 2 of the cassette and into a locking hole 3' formed in the inner face of a cap 3 respectively. The base plate 4 is also provided with two shafts 5 which are respectively inserted into shaft holes 6 formed in the side wall. Further, there is installed a separate torsion spring 7 at one of the shafts 5 in order to provide elastic movements to the base plate 4. The use of such a separate torsion spring 7 necessitates the provision of spring protection ribs 9, 9'. Thus, the construction of the conventional lock mechanism is very complicated, and the expense incurred for procuring and assembling the separate steel torsion spring significantly increases the total cost of the cassette.